edit:
I've uploaded the basic Modular Toolbox framework to GitHub
I've been using Python Toolboxes in production workflows for a while now and have been really irritated with how difficult it is to maintain toolsets that need to be branched between projects.
The whole system is a bit difficult to use with all your tools for a toolbox being in a single pyt file.
I've built a basic system for expanding this functionality using Python's module system and am wondering if there's any interest in the community for me open sourcing the basic framework.
There are some weird bugs still, Arc doesn't reload modules when you refresh a toolbox, so module tools that are updated during production won't get changes until a full Arc reboot. I got around this with explicit reload calls for each tool module in the pyt.
So far it's pretty basic, with a module structure that has toolboxes in the root directory and tools stored in project sub modules within a tool module. (e.g. `tools.<project>.<tool_name>.<tool_class_name>).
There's also a wrapper class for tools that allows you to inherit properties and methods from this base class so you aren't re-implementing the same functions and parameter definitions between tools. So your tools will be defined as `class ToolName(ToolWrapper)` instead of base objects.
So far it's been working well for us and if there's interest from you guys I'd be willing to open source the base framework so other people can start building toolboxes with it.